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Detection of Functional Groups
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Aim:

 

Systematically identify the functional groups in the given organic compound and perform the confirmatory tests after identifying the functional groups.

 

1. Reactions of amides:

 

Action of NaOH:

Amides are decomposed by NaOH to evolve ammonia. The gas can be tested by a moist red litmus paper which is then turned blue.

 

                    Amide                                               Sodium salt of  carboxylic acid       Ammonia

Alkaline hydrolysis of aromatic amides to aromatic acid:

 

The soluble sodium salt of aromatic acid formed from aromatic amides upon hydrolysis is regenerated as white precipitate in acidic medium.

Aromatic amide                            Sodium salt of  Aromatic Acid                         Aromatic acid

Biuret Reaction for aliphatic diamide:

 

When aliphatic diamide is heated at a temperature above its melting point, ammonia is evolved and crystalline biuret is formed. This biuret in alkaline medium gives a violet colour with a drop of copper sulphate solution.

 

Biuret

 

Hydroxamic acid test for aromatic primary amides:

 

Hydrogen peroxide reacts with aromatic primary amides to form the hydroxamic acid, which then reacts with ferric chloride to form ferric hydroxamate complex having a violet colour.

 Aromatic amide

 

Ferric hydroxamate Complex

 

 2. Reactions of carboxylic acids:

 

Reaction with NaOH:Carboxylic acids being acidic dissolves in NaOH to form sodium salt.

 

Solid                                               Soluble Sodium Salt

 

Action of NaHCO3:

 

Forms salt with sodium bicarbonate solution with the evolution of carbon dioxide. 

 

 

Fluorescein Reaction:

 

The anhydrides of aromatic 1,2-dicarboxylic acids on heating with resorcinol gives a dye fluorescein. This dye in NaOH solution gives a yellowish red solution with green fluorescence. 

 


 

 

Anhydride formation:

 

Aromatic 1, 2 dicarboxylic acids decomposes to give its anhydride when heated at its melting point.

 

 

3. Reactions of phenols:

 

Action of neutral ferric chloride solution:

 

Phenol form characteristic coloured iron complexes when treated with neutral ferric chloride solution. Eg phenol & resorcinol - violet colour, catechol-green etc.

 

 

 

Azo dye formation:

 

Aryldiazonium salts react with aromatic rings of phenols to form highly coloured azo compounds. These reactions are called coupling reactions.

 

Benzoylation:

 

Phenols react with benzoyl chloride in presence of NaOH, to form esters.

 

 

4. Reactions of primary amines:

 

Diazotisation:

 

At low temperature (0-5oC) aromatic primary amines dissolved in strong acids (HCl & H2SO4) reacts with nitrous acid (NaNO2 +HCl) to form water soluble diazonium salts.

 

Aliphatic primary amines do not form stable diazonium salts under similar condition. They react with nitrous acid to yield alcohols and nitrogen (causes rapid foaming).

 

Aryl diazonium chlorire                                              

 

Azo dye formation for aromatic primary amines:

 

Aryldiazonium salts react with aromatic rings of phenols to form highly coloured azo compounds. These reactions are called coupling reactions.

 

 

Benzoylation:

 

Primary aromatic amines react with benzoyl chloride in presence of NaOH, replacing the H atom attached to the N atom with the benzoyl group to give anilides.

 

Hinsberg reaction:

 

Hinsberg reagent is benzenesulfonyl chloride.
Primary aliphatic amines on reaction with Benzenesulfonyl chloride & NaOH gives N-alkylsulphonamide which contains an acidic hydrogen and hence dissolve in NaOH solution to form the soluble sodium salt. The solution thus obtained on acidification gives a precipitate of free sulfonamide which is insoluble in HCl.

 

 

5. Reactions of aldehydes:

 

Action of Schiff’s reagent:

 

Schiff’s reagent is a red solution of rosaniline hydrochloride dissolved in water which is decolourised by passing sulphur dioxide.
Dilute solutions of aldehydes when added to Schiff’s reagent restores its red colour slowly.

 

Action of Borsche’s Reagent:

 

Aldehydes reacts with 2,4-dinitrophenyl hydrazine solution to give a orange/red precipitate of aldehyde 2,4-dintrophenylhydrazone derivative.

 

 

 

Action of Tollen’s Reagent:

 

Aldehydes are oxidized to carboxylic acids accompanied by the reduction of silver ions to metallic silver which appears as a mirror under proper conditions.

 

 

Action of sodium bisulphite solution:

 

Saturated solution of sodium bisulphite in water, when mixed with aldehydes gives a white crystalline bisulphite addition compounds.

 

 

6. Reactions of ketones:

 

Action of Borsche’s Reagent:

 

Ketones reacts with 2, 4 dinitrophenyl hydrazine solution to give an orange/red precipitate of ketone 2,4 dintrophenylhydrazone derivative.

 

 

 

Action of sodium nitroprusside solution:

 

The nitroprusside ion, which may be regarded as a special carrier of the nitrosonium ion, forms a coloured complex with methyl ketones. In presence of alkali eg acetone is converted to CH3COCH2- ion which reacts with nitroprusside ion [Fe(CN)5NO]2- to give highly coloured ion [Fe(CN)5NOCH2COCH3]2-.

 

 

 

Identification of functional groups:

    Experiment

     Observation

     Inference

    1. Action of sodium hydroxide solution
     i)  A little of the substance is boiled with dil.NaOH .
     
     ii)Substance is boiled strongly with 20%NaOH solution.      Then cooled & acidified with dil.HCl.

     a. Ammonia is evolved.

     b. Substance  dissolved.

     White crystalline ppt.

     Presence of amides.

     Presence of acidic substances.

     Presence of aromatic amides.

    2. Action of NaHCO3
           To a few ml of the saturated NaHCO3 solution taken in a test tube, a little of the substance is added.

     

     Brisk effervescence with the liberation of CO2.

     

     Presence of acids.

     

    3. Action of FeCl3 solution.
         To a little of the substance in water or alcohol a few drops of neutral FeCl3 solution is added.


      Violet colour.

     

     Presence of phenols.

     

    4.  Action of Schiff’s reagent
           A little of the substance is added to 1ml of the Schiff’s reagent.


      Violet colour developed within 2 minutes.


      Presence of aldehydes.

    5. Action of Borsche’s reagent.
           A little of the substance in methanol is heated with few drops of Borsches reagent in  a water bath.


      A yellowish orange ppt.

     


    Presence of aldehydes or ketones.

    6. Diazotisation
         Dissolve a little of the sample in 2ml con HCl diluted with water& cool in ice. Dissolve sodium  nitrite in water & add the solution dropwise to the cold solution  nitrite in water & add the solution drop wise to the cold solution. 

     

     Rapid foaming.

     

     

    Presence of primary aliphatic amines.

     

     

  1. Confirmatory Tests for Functional Groups:
  2.  

    Tests for aldehydes:

     


      1. Action of Tollen’s reagent:
          A little of the substance is boiled with few drops of Tollen’s  reagent.

     

     Black ppt.

     

    Presence of aldehydes

    2. Action of sodium bisulphite solution:
           Two drops of the aldehyde is shaken with saturated solution of NaHSO3

     

     White crystalline ppt.

     

    Presence of aldehydes.

     

     Tests for ketones:

      

     1. Nitroprusside test:
            Add a few drops of sodium nitroprusside solution to few  drops of ketone. Then add NaOH solution in excess.

     

     Wine red colour.

     

    Presence of methyl ketones.

     

    Test for acids:

     

     1.Fluorescein Reaction:
            A little of the substance is heated with Conc.H2SO4 & Resorcinol in a dry test tube. It is cooled & then poured into a beaker containing excess of NaOH.


      A red solution with intense green fluorescence.


    Presence of dicarboxylic acid.

    2. Anhydride formation:
           A little of the acid is heated in a dry china dish covered with an inverted funnel whose stem is closed. It is then cooled.

     White shiny needles are deposited on the sides of the funnel.

    Presence  of dicarboxylic acid

     

     Tests for amides:

      

    1. Biuret reaction:
             A little of the substance is heated first gently in a dry test tube followed by strong heating. The solid residue is warmed with1 ml 10% NaOH then cooled and one drop of dil.CuSO4 added.


      On heating smell of ammonia is evolved & violet colour on adding CuSO4.

     

    Presence of diamide.

    2 . Hydroxamic acid test:
            Place a little of the substance in 5ml water. Add few drops 3% hydrogen peroxide and 2 drops of 5% ferric chloride. Heat the solution.

     

     Magenta  colour.

     

     

     

     Presence of aromatic primary amide.

     

     

    Test for aromatic amines (primary):

     

1. Add 2ml of cold diazonium solution to a solution of 0.1g 2 -naphthol in 2ml 10% NaOH. & 5 ml water.


2. Benzoylation:
     Dissolved a little of the substance in 10ml 10%NaOH solution contained in a boiling tube.  About 1ml of   benzoyl chloride is added. The boiling tube is corked and shaken vigorously for about 15 minutes.

 

 Orange –red dye.

 

 A white ppt is formed.

 

 Presence of aromatic amines.

 

 Presence of aromatic amines.

     

    Test for aliphatic amines (primary):

     

1.a. To 0.3 mL or 300 mg of unknown substance in a test tube add 5 mL of 10% NaOH solution and 0.4 mL of benzenesulfonyl chloride. Close the test tube with a cork and shake the mixture vigorously. Test the solution to make sure that it is still alkaline using litmus paper.

b. Cool the solution & add 10% HCl solution dropwise.

 

  Soluble in  base.

 

 A precipitate is formed.



Presence of aliphatic primary amines.

 

Presence of aliphatic primary amines.

     

    Tests for aromatic alcohol (Phenol): 

      

1. Azo-dye formation:
         Dissolve two drops of aniline in 1 ml dil.HCl well cooled in ice. Few drops of saturated sodium nitrite solution are added. Then it is added to a well cooled solution of the phenolic compound in aqueous sodium hydroxide.

 

 A red coloured substance is formed.

 

Presence of phenols.

 

2 .Benzoylation:
        Dissolved a little of the substance in 5ml 20%NaOH solution contained in a boiling tube. About 1ml of   benzoyl chloride is added. The boiling tube is corked and shaken vigorously for about 15 minutes.


  A white ppt is formed.


Presence of phenols.

 

 

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